Non-curling reinforced composite membranes with differing opposed faces, methods for producing and their use in varied applications

ABSTRACT

A double-faced PTFE-silicone rubber reinforced composite with curling tendency controlled is achieved by coating one side of a balanced PTFE/glass composite with liquid silicone rubber. Each face of the composite can perform independent functions in a single application, thereby optimizing performance.

BACKGROUND OF THE INVENTION

This invention relates to new and useful reinforced composite membranesfor use in conveying, material handling, surface modifying, surfaceprotection, and barrier applications, in which the two opposing faces ofthe membrane differ significantly in composition and physicalcharacteristics, each face being constructed to perform independentfunctions in a given single application, thereby optimizing overallperformance.

More specifically, this invention relates to reinforced compositemembranes in which one face is a perfluoropolymer, such aspolytetrafluoroethylene (PTFE), and the other face is an elastomer.Although the use of elastomeric materials of varied compositions iscontemplated, silicone rubber is the preferred elastomeric component.

Polytetrafluoroethylene (PTFE) coated fiberglass fabrics and siliconerubber coated fiberglass fabrics are examples of reinforced compositemembranes commonly used in many of the applications mentioned above. Thetwo materials share several unique and valuable physical properties:flexibility, thermal stability in operating environments exceeding 350°F., and low-energy surfaces providing easy release to sticky, viscous,or adhesive materials. On the other hand, they may differ markedly insurface hardness, finish, frictional characteristics, and surfacequalities difficult to specify but related to the way the surfaceadheres to other surfaces. PTFE has one of the lowest coefficients offriction possessed by any common material and exhibits minimal“stick-slip” behavior. On the other hand, silicone rubber, depending onits composition, finish, and hardness (durometer), often has the highcoefficient of friction and pronounced stick-slip behavior or “grabby”quality typically associated with elastomeric materials.

The choice of whether to use a PTFE or a silicone rubber composite in agiven application sometimes involves consideration of the materials'frictional and related surface characteristics. Certain applications mayrequire a material with a low coefficient of friction, in which casePTFE composites would be expected to perform very well, while siliconerubber constructions would not. In other applications, a material with ahigh coefficient of friction or stick-slip characteristics may berequired, in which cases a silicone rubber material would answerreadily, while a PTFE construction would not.

However, in some applications, a membrane with the frictional andrelated surface characteristics of PTFE on one face and those of anelastomer on the other face may be needed. To address this need, effortshave been made to combine the two materials in a double-faced membrane,with PTFE on one face and silicone rubber on the opposite face. In thepast, these attempts have yielded materials with a strong tendency tocurl, making their handling extremely difficult and limiting theirusefulness. The curling tendency is due to imbalanced stresses generatedin manufacturing these composites, the result of differences in thecuring characteristics, thermal coefficients of expansion, and modulaeof the two components. It is the object of this invention to producedouble-faced PTFE-elastomer reinforced composite membranes with curlingtendency controlled to the extent that their handling characteristicsand usefulness remain uncompromised.

SUMMARY OF THE INVENTION

The invention achieves a double-faced PTFE-silicone rubber reinforcedcomposite with curling tendency controlled by coating one side of abalanced PTFE-coated glass composite with liquid silicone rubber. Thecomposite comprises two opposing faces, wherein one face is composed ofa perfluoropolymer, such as PTFE, and the other face is composed of anelastomer, such as silicone rubber. The composite consequently canperform independent functions in a single application, therebyoptimizing performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective and cut away view of an exemplary compositeof the invention.

DESCRIPTION OF THE INVENTION

In accordance with one embodiment the invention, a continuous web of aglass fabric-reinforced composite, with PTFE on one face and siliconerubber on the opposite face, is produced by dip-coating, knife-over-rollcoating, metering and/or wiping, and thermal curing processes.

To produce the membrane in a continuous process, woven reinforcement,which may comprise fiberglass, aramid, or other fiber able to toleratePTFE processing temperatures and suited to the end use of the membrane,is paid off a roll and saturated and/or coated with PTFE by dippingthrough an aqueous dispersion or latex of the resin, removing the excessdispersion by wiping or metering, drying to remove the water, baking toremove most of the surfactant, and finally heating sufficiently tosinter or fuse the PTFE. Typically, the coating operation will berepeated several times in order to apply the desired amount of PTFE freeof cracks and other defects. Alternately, the fusing step is eliminatedin the first several passes, and the multiple layers of unfused PTFEresin applied thus are smoothed and consolidated by passing the webthrough a calendar machine, prior to fusing and the application of oneor more subsequent layers of fused PTFE resin to complete the composite.In any case, the PTFE resin is applied in such a way as to “balance” themechanical forces or residual stresses within the PTFE resin on eachcoating face so that the composite lies flat and does not tend to curl.One way of achieving a balanced-coating is to apply equal amounts ofPTFE to both faces and subject the faces to the same processingconditions. The coating of PTFE should be as thin as possible, whilesufficient to achieve the desired function. Although a variety ofcoating thicknesses of PTFE to serve a variety of functions arecontemplated by the invention, a coating thickness of about 1-5 mils (1mil equals 0.001 in.) is preferred. All the technology involved isfamiliar to those skilled in the art of producing PTFE coated fabricreinforced composites, described for example in U.S. Pat. No. 5,141,800to Effenberger et al., incorporated herein by reference.

In the next step in the process, one face of the PTFE/glass composite isrendered bondable by coating with a mixture of a colloidal silicadispersion, for example DuPont Ludox® 40, and a perfluorinated copolymerresin dispersion, such as fluorinated ethylene propylene (FEP) orperfluoroalkoxy-modified tetrafluoroethylene (PFA). This is applied bydipping, wiping, drying, baking, and fusing, essentially as the PTFEdispersions were applied. Alternately, one face may be rendered bondableby treatment, under appropriate conditions and with appropriate pre- andpost-treatment processing, with a mixture of sodium metal, naphthalene,and a glycol ether, or alternately, a mixture of sodium metal andanhydrous ammonia. If applicable, other means of rendering the PTFEsurface bondable, for example corona treatment in special atmospheres,chemical and electrochemical treatments, metal sputtering, and vacuumdeposition of metals or metal oxides, might be employed. Once again, thetechnology involved in applying bondable treatments to PTFE surfaces isthoroughly described in the literature and is familiar to those skilledin the art of processing PTFE films, articles, and composites of allkinds.

In the final step, the face of the PTFE/glass composite renderedbondable in the previous step is coated with a controlled amount of arelatively low viscosity (ca. 10,000 to 200,000 centipoises) platinumcatalyzed, addition cure, 100 percent solids, solventless, liquidsilicone rubber (LSR) formulation. The LSR coating should be as thin aspossible, only as thick as necessary to achieve the desired function.Although a variety of coating thicknesses of silicone rubber to serve avariety of functions are contemplated by the invention, a coatingthickness of about 2-50 mils is preferred. The LSR formulation iscomposed of commercially available A and B components that are mixed ina specified ratio, typically 1:1 or 10:1. Each component containsvinyl-terminated polydimethylsiloxane polymers and may contain fumedsilica as a reinforcing filler, and/or extending fillers. Typically, onecomponent, for instance the A component, contains the catalyst and the Bcomponent contains a crosslinking agent and an inhibitor that is removedby heating to allow the LSR to cure into a solid rubber. The LSRformulation may incorporate pigments and/or other additives. Themembrane thus coated is completed by passing it through a coating ovenor other heating device, raising the temperature of the coatingsufficiently to drive off or decompose the inhibitor allowing the LSR tocure into a solid rubber.

Alternately, the web may be coated with a silicone rubber incorporatingan organic peroxide catalyst (“heat-curable” silicone) which may beapplied from a solvent solution, dried if necessary to remove thesolvent and heated appropriately to effect a cure, or with siliconerubber incorporating an atmospheric-moisture-activated acetoxy curesystem (“one-package” RTV), once again perhaps from a solvent solution,dried if necessary to remove the solvent and allowed to remain exposedto moist air sufficiently long to effect a cure, perhaps being heated toaccelerate the process.

The resulting composite is a durable, two-faced material with one facePTFE and one face silicone rubber. The composite exhibits flexuralmodulus about that of a plain PTFE/glass fabric composite comprising thesame reinforcement fabric and percent PTFE, and has little tendency tocurl.

Should it be desirable, ribs, lugs, cleats, or other protuberancescomposed of rubber that is the same or similar in composition to therubber face of the composite may be formed on the relatively smoothrubber face by applying beads of flowable, uncured rubber by the use ofrobotically controlled applicators or by methods similar to thosedescribed in co-pending application Ser. No. 09/608,649, filed Jun. 30,2000, the subject matter of which is incorporated by reference herein.The material applied is then cured, depending on its type, according tothe procedures in the paragraphs above. The rubber must be sufficientlyviscoelastic to permit retention of its shape without flowingexcessively during the time required to apply the required number ofcleats and transfer the work to an oven or other heating device or otherenvironment in which the cure is effected.

Unlike other elastomers, silicone rubber has low surface energy.Surprisingly, the surface of silicone rubber is tacky when dry, yet veryslippery when water is present on the surface. This feature of siliconerubber results in unique advantages and end uses which are contemplatedby the invention.

FIG. 1 shows an exemplary composite of the invention in which afiberglass reinforcement (1) is coated on both faces with PTFE (2). Amixture of a colloidal silica dispersion (3) is applied to one surfaceof the PTFE-coated fabric. The application of the colloidal silicadispersion (3) renders the surface bondable. The PTFE/fiberglass facerendered bondable by dispersion (3) is coated with silicone rubber (4,5)to achieve a composite with one face PTFE and one face silicone rubber(5).

EXAMPLE 1

One face of a roll of commercially available PTFE/glass fabric (ChemfabChemglas® Basic 5), containing style 2116 glass fabric as areinforcement and comprised of about 50 percent by weight PTFE resin and50 percent by weight glass, is rendered bondable by applying a mixtureof colloidal silica dispersion (DuPont Ludox® 40), PFA fluoropolymerresin solution (Dupont TE-9946), surfactants, stabilizers, and water;wiping off the excess; drying; baking; and fusing. The fabric weighsabout 5.4 ounces per square yard (osy) and is about 0.005 inches thick.

Using conventional PTFE tower-coating equipment, a coating of an LSRformulation composed of 50 parts by weight each of Wacker SiliconesElastosil® LR6289A and LR6289B and about 12 parts by weight of a rediron oxide masterbatch containing about 35 percent iron oxide and about65 percent vinyl terminated silicone polymer, is applied to the bondableface of the Chemglas Basic 5. The tower is operated to provideconditions of time and temperature sufficient to cause the rubber tocure. The end result is a composite with a smooth, glossy coating ofsilicone rubber on one face and a PTFE surface on the opposite face. Thewhole weighs about 7.5 osy; the silicone rubber coating is about 0.002inches thick and is strongly adhered. The composite lays flat and can behandled easily without curling.

The example composite is fabricated into a belt for a combinationweighing/packaging machine for meat and other food products. In theheat-sealing section of the machine, the non-working face of the belt,i.e., the face that does not contact the product being weighed andwrapped, must slide freely over a heated platen. In another section ofthe machine, the wrapped package must be conveyed up an incline withoutslipping back. An all-PTFE belt slides freely over the heating platenbut allows the package to slip back at the incline. An all-silicone beltcarries the package up the incline without slipping, but does not slidefreely over the heated platen. The example belt functions optimally inboth sections of the machine.

EXAMPLE 2

One face of another roll of the PTFE/glass fabric used in the aboveexample is rendered bondable by treatment with a solution of sodium,naphthalene, and glycol ether. It is coated using the proceduredescribed in Example 1, yielding a composite with physical propertiesalmost identical to those of Example 1.

EXAMPLE 3

One face of a length of Chemltas 64-40916, a Chemfab product comprisedof Style number 64 glass fabric saturated/coated with 40 percent byweight PTFE, is rendered bondable by application of the colloidal silicaformulation as described in Example 1 and coated with 8 osy. of siliconerubber. The resulting product is fabricated into a belt 50 meters longand 1.5 meters wide. It is substituted for a conventionalglass-reinforced silicone rubber belt used as a conveyor and releasesurface in the assembly of plastic wine bags by heat sealing. Thesilicone release surface of the conventional belt performs to the user'ssatisfaction, but the construction is difficult to drive on the user'sequipment due to excessive frictional force generated when the rubbernon-working face of the belt, i.e., the face that does not contact theproduct, slides over stationary components of the machine. The PTFEnon-working face of the example belt generates minimal frictional forceagainst the machine's stationary surfaces, allowing it to be easilydriven, while the working face provides the silicone rubber releasesurface desired.

EXAMPLE 4

A conveyor belt with raised cleats for use in a fast-food-servicetoaster in which the bread products being toasted are slid across aheated platen or griddle by means of force transmitted by the movingbelt, is produced as follows. A rectangle or belt “blank” of appropriatesize is cut from the composite of Example 1. Using a robotic applicator,a pattern of many, identically-shaped raised cleats is laid down on thesilicone rubber face of the blank. The cleats are composed of the sameLSR formulation as the face itself. Each cleat is about 0.8 inches longand roughly simicircular in cross section, about 0.2 inches wide at thebase and 0.04 inches high at the highest point. The longitudinalcenterline of the cleat is a straight line oriented perpendicular to thedirection of travel of the finished conveyor belt. The LSR forming thecleats has viscoelasticity that allows it to retain its shape during thetime it takes to apply the entire pattern of cleats. After the patternis applied, the blank is placed in an oven operating at 500° F. andallowed to remain for two minutes. When the blank is removed from theoven the silicone rubber surface bears a pattern of durable rubbercleats strongly adhered to the surface. The belt is completed byattaching lacings on two opposite ends. When installed on the toasterthe cleats contacting the bread products being toasted, for examplehamburger rolls, are found to drive the rolls more reliably, with lessslippage, than a smooth-faced belt made of similar material.

1. A fiber-reinforced flexible composite comprising: a reinforcementmaterial; a first exposed face on a first side of the reinforcementmaterial, the first exposed face formed from a first material, the firstmaterial having, a low coefficient of friction, and thermal stability inoperating environments exceeding 350° F.; and a second exposed face on asecond side of the reinforcement material opposing the first exposedface, the second exposed face formed from a second material, the secondmaterial having, a high coefficient of friction, and thermal stabilityin operating environments exceeding 350° F.; wherein the flexiblecomposite lies flat and does not tend to curl.
 2. The composite of claim1, wherein the composite comprises about a same flexural modulus when itincludes the second material that it comprises when it does not includethe second material.
 3. The composite of claim 1, wherein the secondmaterial is a portion of a first layer having a thickness of up to about50 mil.
 4. The composite of claim 3, wherein the first layer has athickness of at least about 2 mil.
 5. The composite of claim 1, whereinthe first material is present on the second side of the reinforcementmaterial.
 6. The composite of claim 5, wherein the first material isonly located on the first side of the reinforcement material.
 7. Thecomposite of claim 1, wherein the flexible composite comprises twocompositionally distinct opposing faces; the reinforcement materialconsists essentially of glass fibers; the first material comprises aperfluoropolymer material, the first material being located on each sideof the reinforcement material, the perfluoropolymer in a balanced statehaving mechanical forces within the perfluoropolymer equal on each sideof the reinforcement such that it helps to prevent the membrane fromcurling; and the second material comprises an elastomer disposed overthe first material on one side of the reinforcement.
 8. The composite ofclaim 1, wherein the composite comprises two compositionally distinctopposing faces; the first material comprises perfluoropolymer; thecomposite comprises a first layer comprising the first material and asecond layer of perfluoropolymer material; the reinforcement material isa fibrous reinforcement material and is intermediate the first andsecond layers; the second material comprises an elastomer; the secondmaterial is disposed over the second layer of perfluropolymer material;and the first and second layers have a thickness sufficient to inhibitthe composite from curling.
 9. The composite of claim 1, wherein thecomposite comprises, two compositionally distinct opposing faces; afibrous reinforcement material; and a perfluoropolymer material coatingon each side of the fibrous reinforcement material; the first materialcomprises a perfluropolymer material; the perfluoropolymer materialcoating on each side of the fibrous reinforcement material is in abalanced state having mechanical forces within the perfluoropolymerequal on each side of the reinforcement to prevent the membrane fromcurling; the second material comprises an elastomer; the second materialis disposed over the perfluoropolymer material on one side of thefibrous reinforcement material; and the second material has a thicknessof about 2 to about 50 mils.
 10. The composite of claim 1, wherein thecomposite comprises, two compositionally distinct opposing faces; afibrous reinforcement material; and a perfluoropolymer material coatingon each side of the fibrous reinforcement material; the first materialcomprises a perfluropolymer material; the perfluoropolymer materialcoating on each side of the fibrous reinforcement material is in abalanced state having mechanical forces within the perfluoropolymerequal on each side of the reinforcement to prevent the membrane fromcurling; the second material comprises an elastomer; the second materialis disposed over the perfluoropolymer material on one side of thefibrous reinforcement material; and the weight ratio of thereinforcement to the perfluoropolymer coating is 50:50.
 11. Thecomposite of claim 1, wherein the first material comprises a first layerhaving a thickness of at least 1 mil.
 12. The composite of claim 11,wherein the first layer has a thickness of up to about 5 mil.
 13. Thecomposite of claim 1, wherein the first material comprises a layerhaving a thickness of up to 5 mil.
 14. The composite of claim 1, whereinthe first material and the second material both have low surfaceenergies.
 15. The composite of claim 1, wherein the second face isslippery when water is present and tacky when dry.
 16. The composite ofclaim 1, wherein the second face has pronounced stick-slip and the firstface has minimal stick-slip.
 17. The composite of claim 1, comprisingprotuberances raised above the second exposed face.
 18. The composite ofclaim 1, wherein the composite has a weight of about 7.5 ounces persquare inch (osy).
 19. The composite of claim 1, wherein thereinforcement member consists essentially of glass fiber.
 20. Thecomposite of claim 1, wherein the first material comprises aperfluoropolymer and the second material comprises an elastomer.
 21. Anarticle for modifying surface, comprising: a flexible compositecomprising a reinforcement material; a first exposed face on a firstside of the reinforcement material, the first exposed face formed from afirst material, the first material having, a low coefficient offriction, and thermal stability in operating environments exceeding 350°F.; and a second exposed face on a second side of the reinforcementmaterial opposing the first exposed face, the second exposed face formedfrom a second material, the second material having, a high coefficientof friction, and thermal stability in operating environments exceeding350° F.; wherein the flexible composite lies flat and does not tend tocurl; and wherein the article is configured to be placed on the surfacesuch that the properties of the face of the material not in contact withthe surface are different than the properties of the surface.
 22. Thearticle of claim 21, wherein the first surface of the flexible compositeis configured to be placed in contact with the surface such that thefirst face is not in contact with the surface.
 23. The article of claim21, wherein the first material is formed on the first surface in a layerat least about 1 mil thick.
 24. The article of claim 21, wherein thesecond material is formed on the second surface in a layer up to about50 mil thick.
 25. The article of claim 21, wherein the first materialconsists essentially of polyfluoropolymer and the second materialconsists essentially of elastomer.
 26. The article of claim 21, whereinthe first material is also disposed on the second side of thereinforcement material.
 27. The article of claim 21, wherein the firstmaterial comprises polyfluoropolymer and the second material compriseselastomer; the first material is also disposed on the second side of thereinforcement material; and the second material is not disposed on thefirst side of the reinforcement material.
 28. The article of claim 27,wherein the first material is formed on the first surface in a layer atleast about 1 mil thick; and the second material is formed on the secondsurface in a layer up to about 50 mil thick.
 29. A flexible compositebased belt, the belt comprising: a reinforcement material; a firstexposed face on a first side of the reinforcement material, the firstexposed face formed from a first material, the first material having, alow coefficient of friction, and thermal stability in operatingenvironments exceeding 350° F.; and a second exposed face on a secondside of the reinforcement material opposing the first exposed face, thesecond exposed face formed from a second material, the second materialhaving, a high coefficient of friction, and thermal stability inoperating environments exceeding 350° F.; wherein the belt lies flat anddoes not tend to curl.
 30. The belt of claim 29, wherein the flexiblecomposite comprises two compositionally distinct opposing faces; thereinforcement material consists essentially of glass fibers; the firstmaterial comprises a perfluoropolymer material, the first material beinglocated on each side of the reinforcement material, the perfluoropolymerin a balanced state having mechanical forces within the perfluoropolymerequal on each side of the reinforcement such that it helps to preventthe membrane from curling; and the second material comprises anelastomer disposed over the first material on one side of thereinforcement.
 31. The belt of claim 29, wherein the composite comprisestwo compositionally distinct opposing faces; the first materialcomprises perfluoropolymer; the composite comprises a first layercomprising the first material and a second layer of perfluoropolymermaterial; the reinforcement material is a fibrous reinforcement materialand is intermediate the first and second layers; the second materialcomprises an elastomer; the second material is disposed over the secondlayer of perfluropolymer material; and the first and second layers havea thickness sufficient to inhibit the composite from curling.
 32. Thebelt of claim 29, wherein the composite comprises, two compositionallydistinct opposing faces; a fibrous reinforcement material; and aperfluoropolymer material coating on each side of the fibrousreinforcement material; the first material comprises a perfluropolymermaterial; the perfluoropolymer material coating on each side of thefibrous reinforcement material is in a balanced state having mechanicalforces within the perfluoropolymer equal on each side of thereinforcement to prevent the membrane from curling; the second materialcomprises an elastomer; the second material is disposed over theperfluoropolymer material on one side of the fibrous reinforcementmaterial; and the second material has a thickness of about 2 to about 50mils.
 33. The belt of claim 29, wherein the composite comprises, twocompositionally distinct opposing faces; a fibrous reinforcementmaterial; and a perfluoropolymer material coating on each side of thefibrous reinforcement material; the first material comprises aperfluropolymer material; the perfluoropolymer material coating on eachside of the fibrous reinforcement material is in a balanced state havingmechanical forces within the perfluoropolymer equal on each side of thereinforcement to prevent the membrane from curling; the second materialcomprises an elastomer; the second material is disposed over theperfluoropolymer material on one side of the fibrous reinforcementmaterial; and the weight ratio of the reinforcement to theperfluoropolymer coating is 50:50.
 34. The belt of claim 29, wherein thebelt comprises about a same flexural modulus when it includes the secondmaterial that it comprises when it does not include the second material.35. An apparatus comprising: a machine; and a belt capable of beingdriven by the machine, the belt comprising, a reinforcement material; afirst exposed face on a first side of the reinforcement material, thefirst exposed face formed from a first material, the first materialhaving, a low coefficient of friction, and thermal stability inoperating environments exceeding 350° F.; and a second exposed face on asecond side of the reinforcement material opposing the first exposedface, the second exposed face formed from a second material, the secondmaterial having, a high coefficient of friction, and thermal stabilityin operating environments exceeding 350° F.; wherein the belt lies flatand does not tend to curl.
 36. The apparatus of claim 35, wherein themachine comprises a heated platen; the machine and belt are configuredsuch that the first exposed face will move across the heated platen andthe second exposed face will be in contact with objects.
 37. Theapparatus of claim 35, wherein the machine and belt are configured suchthat the second exposed face will be in contact with objects and thebelt will move the objects along an incline.
 38. A fiber-reinforcedflexible composite comprising: a fibrous reinforcement material; a firstexposed face on a first side of the fibrous reinforcement material, thefirst exposed face formed from a first material comprisingperfluropolymer, the first material having, a low coefficient offriction, thermal stability in operating environments exceeding 350° F.,minimal stick-slip, and a low surface energy; and a layer on a secondside of the fibrous reinforcement material, the layer formed from asecond material comprising a perfluoropolymer, the second materialhaving, a low coefficient of friction, thermal stability in operatingenvironments exceeding 350° F., and a low surface energy; and a secondexposed face, on the second side of the reinforcement material, opposingthe first exposed face, the second exposed face formed from a thirdmaterial comprising an elastomer, the third material having, a highcoefficient of friction, thermal stability in operating environmentsexceeding 350° F., pronounced stick-slip, and a low surface energy;wherein the flexible composite lies flat and does not tend to curl;wherein the composite comprises about a same flexural modulus when itincludes the second material that it comprises when the composite doesnot include the second material; and wherein the composite comprises twocompositionally distinct opposing faces.
 39. The composite of claim 38,wherein the third material is not located on the first side of thereinforcement material; the third material is a portion of a layerhaving a thickness of up to 50 mil; and the first material is present onthe first side with a thickness of 1 mil to 5 mil.
 40. The composite ofclaim 38, wherein the first material and the second material are a sametype of material and the third material is bonded to the secondmaterial.